1. Field of the Invention
The present invention relates to a process for forming an organic semiconducting layer having molecular alignment, and more particularly to a process for forming an organic semiconducting layer having molecular alignment by means of polarized light exposure through a mask.
2. Description of the Prior Art
In recent years, organic semiconducting material has drawn many researchers"" attention and has proven to be one of the most popular candidates in fabrication of thin film transistors (TFTs) and various electronic and optoelectronic devices. Sirringhaus et al. in University of Cambridge use self-organization to produce organic thin film transistor (OTFT) having different anisotropic alignment. It is found that the charge transport efficiency is increased with a better ordered molecular chain. For example, the carrier mobility can be increased by 100 times with better ordered molecular chain. This proves that the molecular alignment of an organic molecule is very important to enhancement of the electrical properties of TFTs. (Nature, Vol. 401, p.685, 1999).
The technology of controlling alignment of an organic molecule can be classified into the following three types.
(1) Self-organization: Sirringhaus et al. produce an organic integrated device including an organic thin film transistor (OTFT) and an organic light emitting diode (OLED). Functional groups in an organic molecule interact with the atom (such as silicon) in a substrate. The interaction provides the organic molecule with better alignment by self-organization. The molecular alignment of organic molecule in the transistor is thus controlled (Nature, Vol. 401, p.685, 1999).
(2) Rubbing or pulling: In U.S. Pat. No. 6,326,640, first, an orientation layer is formed by mechanical rubbing or electric or magnetic field pulling. Next, an organic layer is formed on the orientation layer. In this way, the organic molecule aligns according to the alignment of the orientation layer.
(3) Solvent annealing: In U.S. Pat. No. 6,312,971, an organic semiconducting film is first deposited on a substrate by printing or spin-coating. A specific solvent is selected, such that the alignment of the organic semiconducting molecule is altered using the vapor of the solvent. The electrical properties of OTFT are thus improved.
The above-mentioned conventional technology can only provide the organic semiconducting molecule with the same alignment over the entire substrate. Different alignments in different regions over the same substrate cannot be achieved.
An object of the present invention is to solve the above-mentioned problems and provide a process for controlling the molecular alignment of an organic semiconducing molecule. The present invention can form an organic semiconducting layer with different molecular alignments in different regions over the same substrate. Therefore, the alignment of organic molecule in a transistor channel is accurately defined, thus, the device properties and circuit design are improved. Moreover, since the organic semiconducting layer has different alignments in different regions, it is possible to control the organic molecule to have better alignment in the OTFT channel region, thus providing better carrier transport efficiency during OTFT operation. Similarly, it is possible to control the organic molecule to have worse alignment in the non-channel region, thus providing worse carrier transport efficiency. In this way, current leakage and crosstalk between pixels can be decreased. Also, there is no need to directly pattern the organic material.
To achieve the above objects, the process for forming an organic semiconducting layer having molecular alignment includes the following steps. First, a photoalignment organic layer is formed on a substrate or a dielectric layer. Next, the photoalignment organic layer is irradiated by polarized light through a mask, such that the photoalignment organic layer becomes an orientation layer having molecular alignment. Finally, an organic semiconducting layer is formed on the orientation layer, such that the organic semiconducting layer aligns according to the alignment of the orientation layer to exhibit molecular alignment.
According to a preferred embodiment of the present invention, irradiating the photoalignment organic layer by polarized light through a mask includes the following steps. A predetermined region of the photoalignment organic layer is irradiated through a mask by polarized light having a predetermined direction. The above step of irradiating a predetermined region is repeated at least once, such that the orientation layer formed has at least two molecular alignments in at least two different regions. The polarized lights used in different irradiating steps have the same or different directions. The different irradiating steps irradiate different regions of the photoalignment organic layer.